The present invention relates to an electronic printer having a sheet-supplying unit for delivering a print paper in front of a printer head, and more particularly to a detecting device for detecting the kind of the sheet-supplying unit which is selectively attached to the printer.
A print paper on which characters are printed is required to be loaded on a serial or line printer in either form of a cut sheet and a continuous sheet. The sheet-supplying unit for the cut sheet is called as a sheet-feeler or a cut sheet feeding unit, and a tractor unit is employed for feeding the continuous sheet into and out of the printer. The U.S. Pat. No. 4,489,126 discloses a serial printer with a tractor unit. The tractor unit has a plurality of feed pins which engages the apertures formed on the both sides of the continuous sheet. The sheet is fed by the tractor unit so as to be wound around a cylindrical platen. The tractor unit is intermittently actuated each printing operation is completed to advance the sheet by one line. The platen is driven in synchronism with the tractor unit to feed the continuous sheet together with the tractor unit.
On the other hand, the serial printer disclosed in the U.S. Pat. No. 4,438,915 employs a sheet feeding unit for transporting a cut sheet to the printer. The sheet feeding unit has a sheet stacker for storing piled cut sheets and kick out rollers for picking up one of the sheets and delivering it in front of the printer head. The transported sheet is wound around a platen and pressed on the face of the platen by pressure rollers. The platen intermittently rotates by one printing line after each printing of a line is completed to advance the cut sheet.
It is desired that the printer can print on both the continuous sheet and the cut sheet. To this end, in the serial printer sold under the trademark "SPINWRITER" 3500 series, the two kinds of sheet-supplying units, i.e., the tractor unit and the cut sheet feeding unit are exchangeably provided on the print unit of the printer. The print unit mainly comprises a printing mechanism and a platen for moving the sheet in the printer and is required to feed the sheet in different manner in accordance with the kind of the sheet-supplying unit. Namely, when the tractor unit is attached, the print unit may simply advance the continuous sheet stepwise in response to the printing operation. On the other hand, when the cut sheet feeding unit is attached, the print unit automatically feeds the cut sheet in a certain length at first, such that the first print line of the sheet is transferred at printing position, and then, advances the sheet stepwise in response to the printing operation, and finally, moves the sheet out of the printer after the printing on that sheet is completed.
In order to perform the different control of the sheet feeding, the print unit is necessary to recognize the kind of the sheet-supplying unit attached thereto. Heretofore, the tractor unit and the sheet feeding unit have had an electrical signal line, respectively, to be connected to the print unit of the printer, through which an electrical signal is transmitted to the print unit for informing the print unit of what kind of sheet-supplying unit is being used. Accordingly, the signal line must be connected electrically to the print unit when the tractor or cut sheet feeding unit is mechanically connected to the print unit. Therefore, exchanging operation of the sheet-supplying unit is complicated.
Further, the sheet-supplying unit is required to detect the absence of the sheet so as to prevent the printer from printing on a bare platen. In the prior art printer, each of the tractor unit and the cut sheet feeding unit has a mechanical switch or an optical sensor for detecting the absence of the sheet. The detector signal is supplied to the print unit via the signal line connected to the print unit. The necessity of the mechanical switches or the optical sensors on the two sheet-supplying units increases the manufacturing cost of the sheet-supplying units and decreases the durability of the same owing to breaking down of the switch or the sensor.